This DOES NOT have to be done during a single run.
When:
Anytime from the start of the game.
Method:
Across most of the maps you'll find torches, candles, braziers etc. These can be destroyed by your weapons, and will leave behind coins or a roast chicken. Collect FIVE of these chickens across all your games (can easily be done in one) and you'll unlock the Garlic, a very useful weapon for beginners.